It is known that in spot welding plants, with use, the electrodes of the welding guns get worn, deformed and contaminated with metallic residues and oxide deposits. Therefore, the electrodes have to be frequently redressed to regain a clean and proper surface. As known, it is necessary to remove a certain thickness of contaminants that form on the electrode surfaces. The thickness of the contaminant deposit affects the overall electrical and thermal conductivity of the electrode. The shape and size of the electrode, in addition, have a determining influence on the size of the so-called “lens” of the weld. If the electrode tip is too wide, adequate fusion may not be achieved at the interface between the surfaces of the two metal sheets to be welded together.
Typically, when the electrodes exceed a certain size, the operator compensates by increasing the intensity of the electrical current passing through the electrodes. The increased current leads to a number of unfavorable factors. First, the current increase causes an overheating of the cooling liquid and a subsequent enlargement of the welding electrode tips. Overheating causes a reduction of the electrode hardness and a deterioration of its mechanical characteristics. Along with this, a phenomenon known as “mushrooming” occurs, i.e. an increase in the diameter of the electrode and, with it, also the deposition of contaminants.
A need is felt in this technical field to optimize the removal of material deposited on the electrodes. Heretofore, dressing processes have been carried out at regular intervals, according to standardized parameters, concerning the duration of the dressing phase and the clamping force with which the welding gun closes its electrodes on the dressing unit. Standard parameters, in terms of time and force, represent a compromise that is suitable in most cases. However, the removal of material occurs without a knowledge of the exact thickness of the surface layers of electrodes that should be removed.
Further, dressing has heretofore been carried out in an undifferentiated manner to both the anode and the cathode. Since a greater deposit of contaminants occurs on the cathode, in order to optimize electrode duration and operation, deposits should be removed to a greater extent from the cathode than from the anode.